Here [0] you can find a decent summary of the book. For me the single most important point that the book taught me was that it's ok to write in books, they don't have to be in pristine condition without any text in them. The white pages in books are meant specifically to write on. They explain this point of the book in a separate blog post [1] for some reason. Anyway... I never used to write in books, now this has become a central part of my learning process. The physical act of scribbling a question mark next to unclear parts, rephrasing what is meant and so on has helped me understand complex material faster as well as made it easier to refer to at a later point (as there is no need to worry about any notes getting lost).

[0] https://fs.blog/how-to-read-a-book/ [1] https://fs.blog/2015/01/marginalia/